We propose to study visual processing in a novel manner, by isolating zebrafish mutants on a large scale. In particular, we are interested in the segregation of visual functions in the brain, a well-known but poorly-understood feature of vertebrate vision. Different kinds of visual information are processed by highly specialized neurons in the retina and channeled into separate brain nuclei. These pathways in turn generate distinct behavior patterns in response to visual stimulation. We propose to establish a functional map of the zebrafish visual brain by using genetic lesions as a dissecting tool. Six different behavioral assays will be used to search for mutations affecting specific visual functions in a screen modeled after the highly successful screen for mutants in the retinotectal projection. We will test for optomotor responses and optokinetic responses to moving gratings, and for adaptation of skin pigment to ambient light levels (a process requiring retinal input). The optomotor assay will be combined with two motion- nulling paradigms (borrowed from human psychophysics), which test for the intactness of color and luminance channels. Our pilot screens predict that this large-scale approach will uncover a broad assortment of several hundred mutants. The specificity of the phenotype will vary with the site and extent of the genetic lesion. Many phenotypes will allow us to assign functions to certain nuclei, pathways, or cell types. Some mutants will remain puzzling, particularly those with no detectable anatomical disruption. As a tool for the screen, mutagenized males will be crossed into a line of transgenic fish, which express green fluorescent protein in retinal axons. This genetic background will later permit the rapid detection of mutations in retinorecipient areas. In parallel to the mutant screen, we will investigate the types of retinal ganglion cells and their axonal projections by DiI tracing, both in wildtype and in selected mutants. Health-relatedness. Given the large degree of conservation of genes between fish and humans, this approach directly addresses the problem of inherited human blindness, and leads a way to isolating the genes involved in diseases of the human retina and central visual pathways.